Oxidative stress, a hallmark of inflammation, is potentially a major cause of the vascular injury leading to atherosclerosis. Interaction between extracellular matrix (ECM) components and cell adhesion receptors activate intracellular signaling pathways that regulate leukocyte and vascular cell growth and function. We hypothesize that oxidative stress leads to modifications of ECM components and cell adhesion receptors that promote atherogenesis. Hyaluronan (HA) is a component of ECM that accumulates in atherosclerotic lesions. Our preliminary data demonstrate that CD44, the principal receptor for HA, promotes atherogenesis in mice by mediating inflammatory cell recruitment and activation of vascular cells. The ubiquitous, high molecular weight (HMW) form of HA can be modified directly by enzymatic or oxidative cleavage at sites of inflammation that leads to accumulation of lower molecular weight fragments of HA (LMW-HA). LMW-HA exhibits proinflammatory activities on macrophages and vascular smooth muscle cells (VSMC) while HMW-HA may play a critical role in inhibiting activation of leukocytes and VSMC. We propose to test the specific hypothesis that oxidative stress leads to modifications of HA and CD44 that promote atherogenesis. Specifically, in Aim 1 we will determine the capacity of reactive oxygen (ROS) and reactive nitrogen (RNS) species to modify HA and test the hypothesis that oxidative modification of HA effects its: i) ability to bind to CD44; ii) internalization and degradation by various cell types; and iii) susceptibility to degradation by hyaluronidases. The in vitro studies will be complemented by comparing the levels and structure of HA in aorta of normal mice and in atherosclerotic lesions in apobec-l/LDLR double knockout mice (DKO). In aim 2 we will test the hypothesis that oxidative stress regulates the expression and activation of CD44 on leukocytes and vascular cells and characterize the underlying structural modifications of CD44 both in vitro and in vivo. In aim 3 we will test the hypothesis that modifications induced by oxidant stress alters the response of macrophages and VSMC to HA and regulates the balance between the inflammatory and fibrotic responses in atherosclerotic lesions. Experiments proposed in Projects 1 and 5 will extend these studies by determining whether the regulation of HA and CD44 correlates with markers of oxidant stress induced by smoking and nicotine and whether the anti-oxidant and anti-inflammatory effects of HDL and its components such as apoAl correlate with regulation of HA and CD44 in mouse and man. Results from these studies could lead to the identification of targets for novel therapeutic strategies to stop progression or induce regression of atherosclerotic disease or to regulate the inflammatory response to promote morphologic changes associated with stabilization of lesions.